HomeMy WebLinkAbout839 CAMPFIRE DR - SPECIAL INSPECTIONS - 6/26/2012♦A
A.G.
enaar
Geotechnical and Environmental Consultants
D R Horton
9555 South Kingston Court, Suite 200
Englewood, Colorado 80112-5943
Attention: Mr. Kyle S. Gunther
Subject: Soil and Foundation Study
Proposed Residential Structure
Lot 18, Block 4
Trail Head
Fort Collins, Colorado
Project Number 120922
Purpose
2180 South Ivanhoe Street, Suite 5
C)enver, Colorado 80222-5710
303-759-8100 Fax 303-756-2920
www.agwassenaar.com
June 26, 2012
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As requested, we have performed a soil and foundation study at the subject site. The purpose of our
study was to observe subsurface conditions encountered and to recommend geotechnical design criteria
for the design and construction of the foundation for the proposed residence. This letter presents a
summary of our findings and recommendations.
Subsurface Conditions
The field exploration included drilling a 4-inch diameter auger boring near the center of the lot to a depth
of approximately 20 feet. The subsurface materials encountered consisted of:
0' to 6' Fill, clay, stiff, silty, sandy, moist, mottled brown to dark brown
6' to 20' Clay, stiff, silty, sandy, slightly moist to moist, brown to dark brown
Date of drilling: May 8, 2012
Depth to water: Dry at the time of drilling
Dry 6 days after drilling
Laboratory Testing
Samples obtained during drilling were returned to the laboratory. They were visually classified and testing
was assigned to selected samples in an effort to evaluate the engineering properties of the subsurface
materials encountered. Site specific laboratory swell/consolidation tests exhibited low measured swell
(0.7% at a depth of 4 feet and 0.4% at a depth of 9 feet) upon an increase in moisture content under a
load of 1,000 pounds per square foot (psf). Based upon visual observation of the subsurface conditions
encountered and laboratory testing for this and/or adjacent lots, it is our opinion that the subsurface
materials generally exhibit moderate potential for expansion. Refer to the Colorado Geological Survey
Special Publication 43 for a description of expansive soils and their impact on structure performance.
D R Horton
Project Number 120922
June 26, 2012
Page 2
Foundation Recommendations
Based on our evaluation of the subsurface conditions, the proposed residence may be founded upon
spread or pad -type footings bearing on the natural undisturbed soils or on properly placed and compacted
fill at basement depth. If footings will be placed at normal depth for a crawl space, garage and/or
porch, a soil modification process to basement footing level or to a minimum depth of 6 feet
below bottom of crawl space, garage and/or porch footings will be required.
Soil Modification
Overexcavation or another soil modification process will be necessary in order to utilize a spread or pad
type foundation for non -basement areas (crawl space, garage, and/or porch). The following
recommendations should be followed during overexcavation operations in order to enable the placement
of a moisture treated fill that can be used for foundation support. These recommendations may be
modified during construction if soil conditions differing from those anticipated are encountered.
The existing soil should be excavated to a depth of at least 6 feet below the bottom of the
lowest foundation element on the lot. The base of the excavation should extend to a width
of at least 5 feet beyond the potential foundation footprint (including any counterforts). The
excavation should be sloped following current OSHA regulations. A licensed surveyor must
verify the depth and width of the excavation prior to any fill placement.
Once the excavation limits have been verified, fill placement may begin. The bottom of the
excavation should be scarified and moistened prior to fill placement. The replacement fill
should be placed in maximum 8-inch, loose lifts. If the replacement fill consists of cohesive
soils, the fill should be moistened and thoroughly mixed to obtain a moisture content
between 0% and +4% of optimum moisture content and then compacted to a minimum of
95% of ASTM D 698 maximum dry density. If the replacement fill will consist of granular
soils, the fill should be moistened to a moisture content of ±2% of optimum moisture content
and then compacted to a minimum of 95% of ASTM D 1557 maximum dry density. It is
recommended that a grading compactor suited for such a fill be utilized to aid in compaction
of the fill. Optimum moisture content and maximum dry density should be obtained from
appropriate laboratory Proctor tests.
3. Observation and testing of fill placement must be performed by this firm on a full-time basis.
Testing should include in -place moisture content and dry density as well as periodic swell -
consolidation testing.
4. Placement and compaction of fill should continue to final overlot grade. We do not
recommend that placement of fill stop at foundation elevation as drying of the near surface
fill prior to foundation construction may allow sufficient heave after construction to distress
the residence. This may be waived if the foundation will be constructed within one month
of completion of fill placement.
D R Horton
Project Number 120922
June 26, 2012
Page 3
The subsurface conditions must be evaluated at the completion of fill placement by
conducting additional test borings, sampling, and testing. This requirement may be waived
if a representative of our firm observes and tests the fill on a full-time basis.
Alternative soil modification processes, such as water injection, may be utilized if overexcavation is not
desired, upon approval by our office. The selected process must be capable of reducing the potential for
expansion of the clay soil to a level suitable for support of footing foundations, as determined by our office.
Depending upon the process selected, additional testing during or upon completion of the processes will
be necessary to confirm the results of the modification meet the intent. Additional recommendations will
be made upon selection of a modification process.
It must be understood that while these methods are used to reduce the likelihood of future heave, it is not
free of risk of foundation movement. While future heave is less likely, the possibility of moisture induced
settlement is increased. Therefore, the control and removal of surface water at the site will continue to
be very important.
Provided all footings for the structure are at basement level or upon completion of the soil modification
process, the footings should be designed for a maximum bearing pressure of 2,500 psf with a minimum
dead load pressure of 800 psf. Four -inch void material should be installed in areas where the minimum
dead load cannot be attained. Footing dimensions and foundation structural elements should be
determined by a structural engineer. Concrete in contact with the subsurface materials should be
designed for very severe sulfate exposure as defined by ACI 318.4.3. Bearing materials loosened by
machine excavation should be removed prior to placing footing concrete. Occasionally, pockets of dry,
hard fill or very moist, soft fill may be encountered in the foundation excavation. If this condition occurs,
the footings should extend to properly moisture treated fill. Exterior footings should bear at least 3 feet
below exterior grade forfrost protection. The bearing materials beneath footings should be protected from
freezing during construction. All footing excavations should be observed prior to placement of concrete
to confirm the footings are bearing on suitable materials as anticipated for design purposes.
The foundation walls backfilled with on -site materials should be designed for a lateral earth pressure
based upon an equivalent fluid density of 55 pounds per cubic foot (pcf) for the "at rest' condition or 45
pcffor the "active" condition. The "active" condition should only be used where wall movements of at least
0.5% of the wall height are allowed. These values have been provided without considerations for sloping
backfill, surcharge loading or hydrostatic pressures. Construction of a drain system and proper surface
drainage as discussed later in this report may lower the potential of developing hydrostatic pressure in
the backfill materials. Minor cracking of concrete foundation walls should be expected.
Basement Floor Construction
A basement slab performance risk evaluation was conducted in general conformance with industry
guidelines for the local area. The risk assessment of a site for potential movement is not absolute; rather,
it represents a judgment based upon the data available and our experience in the area. Movement of
foundations and concrete flat work will occur with time in low to very high risk areas as the soil moisture
content increases. On low and moderate rated sites, slab movements of up to 3 inches across the slab
with slab cracking of up to '/4 inch in width and/or differential are considered normal. The damage
generally increases as the risk assessment increases and as the depth of wetting increases. It must be
D R Horton
Project Number 120922
June 26, 2012
Page 4
understood, however, that assessing risk is an opinion. There is currently no type of testing or correlation
of factors that will definitively predict the amount of heave that a floor slab will exhibit. Therefore, it may
be possible that heaves less than or in excess of what is considered "normal" may be experienced.
For sites with a risk assessment of high or very high, we recommend an interior floor system engineered
for expansive soils be constructed. An alternative to the use of an engineered floor system, such as soil
modification to reduce the risk assessment, may also be considered. In addition, an engineered interior
floor system is recommended for all finished areas or any other areas where floor movements cannot be
tolerated.
Based upon our evaluation of the subsurface conditions at this site, it is our opinion that the slab
performance risk for this site is low. If this risk of movement is not acceptable, engineered interior floors
should be constructed or an alternative such as soil modification should be considered.
If the Builder and/or Owner desires to construct a concrete slab -on -grade and accepts the risk of slab
movement, slabs supported by the expansive subsurface materials should be constructed using the
following criteria:
Slabs should be separated from exterior walls and interior bearing members with a
joint which allows free vertical movement of the slab.
2. Slab bearing partitions should be constructed with a minimum 2-inch void space.
Stairways bearing upon the slab should be constructed in such a way as to allow at
least 2 inches of slab heave. In the event of slab heave, the movement should not be
transmitted directly through the partitions to the remainder of the residence.
3. Plumbing and utilities should be isolated from the slab.
4. Where a forced -air heating system is used and the furnace is located on the slab, we
recommend provision for a collapsible connection between the furnace and the duct
work to allow for at least 3 inches of slab heave. Utility connections should also be
provided with flexible connections capable of accommodating the same magnitude of
movement as specified above.
5. Provide frequent control joints in the slab.
Following these recommendations will reduce immediate damage caused by movement of the floor slab;
however, the void spaces recommended are not intended to predict total slab movement. Care should
be taken to monitor and reestablish partition voids and flexible connections when necessary. We are
available to provide further consultation regarding basement slab performance risk assessments.
Crawl Space Construction
The crawl space ground surface should be sloped to the perimeter drain system. Trenching or dishing
out of the crawl space is not recommended unless a drain system is placed in these areas in such a
manner to facilitate drainage. The recommended clearance from the crawl space ground surface to the
D R Horton
Project Number 120922
June 26, 2012
Page 5
engineered floor system should meet applicable codes as well as be increased by the recommended
foundation void height. In addition, all plumbing lines should be isolated from the ground surface or
foundation walls by at least the height of the previously recommended void thickness.
During construction, the crawl space area should be checked for standing water or very moist conditions,
construction debris, and other deleterious materials. If these conditions exist, the area should be
evaluated and mitigated, as necessary.
Crawl space areas should be constructed with consideration given to proper ventilation and moisture
management. Provisions such as the installation of a vapor retarder should be utilized to reduce the
amount of moisture (humidity) in the crawl space air. The Client and any future Owner should be aware
that crawl space areas are subject to various air quality issues. A consultant specializing in ventilation
and air quality control should be contacted to provide any additional recommendations. Such
recommendations are beyond the geotechnical scope of this study. The environmental division of A. G.
Wassenaar, Inc. is capable of providing such services. Refer to "Homeowner's Guide To Moisture
Management" by Tri-County Health Department (Brochure Number S-323) for additional information.
Subsurface Drainage
As a minimum, we recommend providing a subsurface drainage system around the lowest below grade
area. The purpose of the drain is to collect water which may become trapped on the surface of the
excavation and enter the basement or crawl space areas. A drain should be constructed similar to the
attached drain detail (Figure 1) and should be uniformly sloped to a positive gravity discharge or sump.
If a sump pit is installed, it should be monitored for water accumulation and proper operation. The water
level in the sump pit should not be allowed to rise above the foundation drain inlet pipe(s). If water rises
above the inlet pipe(s), a pump should be installed (if not originally equipped) or maintenance should be
performed on the existing pump.
Surface Drainage
The wetting of foundation soils and/or bedrock materials which causes heave may be reduced by carefully
planned and maintained surface drainage. The following recommendations should be implemented
during construction and maintained by the Homeowner after the residence is completed:
1. Excessive wetting or drying of the open foundation excavation should be avoided as
much as practical during construction.
2. The ground surface surrounding the exterior of the foundation should be maintained
in such a manner as to provide for positive surface drainage away from the foundation.
At completion of construction, we recommend a minimum fall awayfrom the foundation
of 6 inches in the first 5 feet. This slope should be continuous across the backfill zone.
3. Backfill around the foundation should be moistened and compacted in such a manner
as to reduce future settlement. Areas which settle should be filled as soon as possible
in order to maintain positive drainage away from the foundation.
D R Horton
Project Number 120922
June 26, 2012
Page 6
4. If lawn edging is used around the exterior of the foundation, it should be constructed
in a manner to prevent ponding of surface water in the vicinity of the backfill soils.
5. All drainage swales should be constructed and maintained a minimum of 5 feet away
from the foundation on side yards and 15 feet away from the foundation on back and
front yards. Drainage swales should maintain a slope of at least 2% off of the lot.
Swales must not be blocked by fences, landscaping, paths or other Homeowner
installed items.
6. Roof downspouts and drains should discharge well beyond the limits of foundation
backfill.
7. Watering adjacent to the foundation should be reduced as much as practical.
Landscaping which requires excessive watering should not be located within 5 feet of
foundation walls. Main sprinkler lines, zone control boxes and drains should be
located outside the limits of the foundation backfill. Sprinkler heads should be
positioned such that the spray does not fall within 5 feet of foundation walls.
8. Plastic membranes should not be used to cover the ground surface immediately
surrounding the foundation. These membranes tend to trap moisture and prevent
normal evaporation from occurring. We recommend the use of a weed suppressant
geotextile fabric.
Limitations
We believe the professional judgments expressed in this reportare consistentwith that degree of skill and
care ordinarily exercised by practicing design professionals performing similar design services in the
same locality, at the same time, at the same site and under the same or similar circumstances and
conditions. No other warranty, express or implied, is made. The location of the test boring drilled and the
laboratory testing performed for this study were designed to obtain a reasonably accurate picture of
subsurface conditions for design purposes. Variations in subsurface conditions not indicated by the
boring are possible and expected. Therefore, we should be retained to observe the foundation excavation
and construction in order to verify or revise our recommendations. If unexpected subsurface conditions
are observed by others during construction, we should be called to review our recommendations.
This reportwas prepared for the exclusive use of our Client for the sole purpose of providing geotechnical
design criteria for the subject structure based upon the existing site conditions as encountered. The
conclusions and recommendations contained in this report shall not be considered valid for use by Others
without written authorization from A. G. Wassenaar, Inc. In addition, the state of practice in geotechnical
engineering is constantly evolving. Therefore, findings presented in this report should be reviewed and
revised, if necessary, prior to actual construction.
The recommendations provided in this report are based upon the specified extents of the overexcavation
for the original building footprint. The future Homeowner(s) should not construct any additions to the
residence utilizing the recommendations given in this report. Additional studies must be provided if any
additions are to be constructed.
D R Horton
Project Number 120922
June 26, 2012
Page 7
If we can be of further service in discussing the contents of this letter or in analysis of the proposed
structure from the soil and foundation viewpoint, please call our office.
Sincere) �\\0WII111II1//!!//
Y, \PO0°REr,/S///
A. G. WASSENAAR, IO¢�
Robert U. BraKson, P.
Senior Engineer
Reviewed by:
Keith D. Seaton, P.E.
Senior Engineer
RUB/KDS/lia
Attachment: Figure 1
Statement of Services
A.G. Wassenaar
Geotechnical and Environmen�Consult�ants�C_�
NOTES:
1. DRAIN MUST SLOPE TO A POSITIVE
GRAVITY OUTLET AND/OR TO SUMP
WHERE WATER CAN BE REMOVED
BY PUMPING
2. SLOPE BOTTOM OF TRENCH AND PIPE AT A
MINIMUM OF 1/8" PER FOOT (i.e. 1 %)
OR AS APPROVED BY THE GEOTECHNICAL ENGINEER
3. 4-INCH DIAMETER RIGID PERFORATED PVC
PIPE (ASTM D2729 MINIMUM SCHEDULE 20), OR
SUBSTITUTE APPROVED BY GEOTECHNICAL ENGINEER
4. GRAVEL SPECIFICATION: 2" MINUS WASHED
ROCK/GRAVEL, POORLY GRADED WITH NO MORE
THAN 30% PASSING THE 3/8" SIEVE AND NO
MORE THAN 10% PASSING THE #4 SIEVE, OR AS
APPROVED BY THE GEOTECHNICAL ENGINEER
SLIP
JOINT
CONCRETE SLAB ON GRADE
BACKFILL
DO NOT EXCAVATE WITHIN A 1:1 LINE —
EXTENDING DOWN AND AWAY FROM
EDGE OF FOOTING. MAINTAIN THIS ANGLE
OF EXCAVATION TO A DEPTH OF TWICE THE
FOOTING WIDTH.
FOUNDATION MINIMUM 6 MIL POLYETHYLENE BARRIER OR
WALL EQUIVALENT APPROVED BY GEOTECHNICAL
ENGINEER. ATTACH TO WALL AFTER
DAMPPROOFING AND EXTEND AT LEAST 1
FOOT UP ON WALL AND BENEATH FLOW LINE
OF PIPE.
BACKFILL
NON -WOVEN GEOTEXTILE FILTER
FABRIC (MIRAFI 140N OR EQUIVALENT
APPROVED BY GEOTECHNICAL
ENGINEER) PLACED ACROSS ENTIRE
WIDTH OF DRAIN GRAVEL.
DRAIN GRAVEL (SEE NOTE #4) GRAVEL
SHOULD FILL ENTIRE TRENCH AND EXTEND
LATERALLY TO TOP OF FOOTING. GRAVEL
COVER ABOVE PIPE SHOULD BE AT LEAST 6"
8"
MINIMUM
FOOTING FOUNDATION
D.R. HORTON-NORTH TYPICAL EXTERIOR DRAIN DETAIL
FTG 3 EXT-SOG FIGURE 1
SEPTEMBER 2007
EXTEND POLYETHYLENE TO OUTSIDE
EDGE OF BOTTOM OF TRENCH.
DRAIN PIPE - (SEE NOTES #2 & 3)
AT LOCATION OF HIGH POINT,
ESTABLISH BOTTOM OF DRAIN
PIPE AT OR BELOW
BOTTOM OF FOOTING OR PAD